Turning Up the Heat on Citrus Greening

It’s like a sauna for citrus, but this is no luxury – and neither is time – for Florida growers. An innovative new approach to the long-standing plague of citrus greening employs steam to treat trees infected with the crop-wasting bacteria.

Pioneered by Reza Ehsani, Ph.D., associate professor of agriculture and biological engineering at the University of Florida Citrus Research and Education Center in Lake Alfred, Florida, citrus steam treatments seem to be the best hope for securing this vital crop worth an estimated $9 billion to Florida’s economy. And given citrus greening’s rapid acceleration, this breakthrough comes not a moment too soon.

“There’s an exponential curve where the first phase of this disease is slow, but we are entering the second phase where we are losing crops at a faster rate,” Ehsani said. “There is a sense of urgency among the growers to do something now because it may be too late if we wait more.”

As of December 2014, Ehsani and his team had applied this thermotherapy to 1,200 citrus trees in multiple Central Florida groves. Stressing that the long-term picture of citrus greening remains cloudy, the hot vapors appear to, at least, cleanse the emergent portions of infected trees.

“I’m trying to be careful not to make any claims that are not true,” Ehsani said. “It takes time to collect data, but short-term results are encouraging.”

The Problem

Backing up a step, citrus greening was discovered in southern China in the early 1900s. Also known as huanglongbing (HLB), the scourge was first found in Florida in 2005. Transported tree to tree by a winged insect known as the Asian citrus psyllid, citrus greening starts with leaf discoloration and progresses into branch dieback and discolored, misshapen fruit unfit for consumption.

Citrus steam treatments seem to be the best hope for securing this vital crop worth an estimated $9 billion to Florida’s economy. And given citrus greening’s rapid acceleration, this breakthrough comes not a moment too soon.

As Ehsani explained, controlling the psyllid is very difficult. Spraying has limited effects, and with a 2 percent survival rate, the insect can quickly reestablish. And once the infection reaches the tree’s roots, the damage is irreversible.

“Besides the steam treatment, there’s no way to kill the bacteria,” Ehsani said. “If you can’t kill the bacteria, you have to remove the bacteria, and if you remove the bacteria you put yourself out of business. Until we can develop a tree that is resistant to the bacteria, we have no other option.”

Fitting the hydraulic arms of citrus hauling trucks with a tenting device, Ehsani’s team is able to quickly cover a tree like an awning. Then, using a commercial-grade steam generator, they inject a very wet steam into the tent and bring the temperature up to about 137 degrees Fahrenheit.

Elsewhere, some growers have tried treating infected trees with solar-heated tents for up two days at a time. Seasonal temperatures and sunlight periods yield varying results, while the labor-intensive process is slow and costly. By comparison, Ehsani’s technique works year-round.

Moreover, with no chemical use, the steam treatments require no regulation. Steam is environmentally friendly and works for multiple types of citrus.

As with any new method, Ehsani noted that his steam technique is still a work in progress. He has applied for additional grant funding to continue his research and refinement.

“We’re trying to understand how the tree responds in various seasons,” he said. “It’s possible to use this treatment year-round, but we’re trying to understand the best time of year.”

The Outlook

Though thermotherapy has had promising outcomes, it is not without its drawbacks. According to a report by the University of Florida’s Institute of Food and Agricultural Sciences, heat-treated trees often drop their old leaves and the new ones are easy psyllid targets. That makes reinfection a strong likelihood, but Ehsani said thermotherapy remains the best tool in the box.

“This is not a cure; it is just a short-term [action] to prolong the life of the tree,” he said. “It gives growers time to stay in production and make money and not lose their crops.”

On the upside, Ehsani is pleased with the private-sector interest his work has sparked. Several manufacturers, including ScoringAg.com, have made progress on commercial citrus steam treatment equipment. Moreover, grant money available through the U.S. Department of Agriculture will likely enable further development.

“I’m very encouraged because really, there is no other option in the short term,” he said. “There’s a lot of long-term research into controlling the psyllid and growing a [more resistant] citrus tree. The steam treatment, in a way, is a simple solution to a difficult problem.

“I think with the amount of money that is being put into this, we will see a cure at some point in the near future. The problem we have is that we don’t have enough time.”

Suffice it to say that this is cause sufficient to turn up the heat on citrus greening.

Caption for top photo: Trees infected with citrus greening, but not treated with heat, have obvious disease symptoms and reduced productivity. Credit: Photo by Marco Pitino

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